Feedback is a well known problem in hearing aids and several systems for suppression and cancellation of feedback exist within the art. With the development of very small digital signal processing (DSP) units, it has become possible to perform advanced algorithms for feedback suppression in a tiny device such as a hearing instrument, see e.g. U.S. Pat. Nos. 5,619,580, 5,680,467 and 6,498,858.
The above mentioned prior art systems for feedback cancellation in hearing aids are all primarily concerned with the problem of external feedback, i.e. transmission of sound between the loudspeaker (often denoted receiver) and the microphone of the hearing aid along a path outside the hearing aid device. This problem, which is also known as acoustical feedback, occurs e.g. when a hearing aid ear mould does not completely fit the wearer's ear, or in the case of an ear mould comprising a canal or opening for e.g. ventilation purposes. In both examples, sound may “leak” from the receiver to the microphone and thereby cause feedback.
However, feedback in a hearing aid may also occur internally as sound can be transmitted from the receiver to the microphone via a path inside the hearing aid housing. Such transmission may be airborne or caused by mechanical vibrations in the hearing aid housing or some of the components within the hearing instrument. In the latter case, vibrations in the receiver are transmitted to other parts of the hearing aid, e.g. via the receiver mounting(s).
WO 2005/081584 discloses a hearing aid capable of compensating for both internal mechanical and/or acoustical feedback within the hearing aid housing and external feedback.
It is well known to use an adaptive filter to estimate the feedback path. In the following, this approach is denoted adaptive feedback cancellation (AFC) or adaptive feedback suppression. However, AFC produce biased estimations of the feedback path in response to correlated input signals, such as music.
Several approaches have been proposed to reduce the bias. Classical approaches include introducing signal de-correlating operations in the forward path or the cancellation path, such as delays or non-linearities, adding a probe signal to the receiver input, and controlling the adaptation of the adaptation of the feedback canceller, e.g., by means of constrained or band limited adaptation. One of these known approaches for reducing the bias problem is disclosed in US 2009/0034768, wherein frequency shifting is used in order to de-correlate the input signal from the microphone from the output signal at the receiver in a certain frequency region.